Voltage controlled oscillators (VCOs) are well known and widely used in the electronics industry Within the digital communications field, VCOs are used in a variety of applications. Such applications include, for example, frequency synthesizers, signal generation, (e.g., serial transmission clock recovery) and the like. VCOs are typically designed to perform within a given set of boundary conditions and to perform to a specified standard. Typical conditions include, for example, performance over operating temperature ranges, sensitivity to vibration, output sensitivity to interference, and the like. Typical performance standards include, for example, output signal frequency stability, output signal programmability, and the like.
A typical prior art VCO generates an oscillating output signal having a specified frequency. The signal can have several different wave forms (e.g., square, saw tooth, triangular, etc.). The frequency of the output is tunable and is a function of an input voltage, an external resistance or capacitance, or the like. The type of application in which the VCO is used dictates its operating conditions and performance requirements. However, as in most VCO applications, it is usually important that the output frequency of the VCO is stable and is a consistent function of the control inputs (e.g., voltage, capacitance, and the like). The output frequency should also be stable with respect to the different process corners of the fabrication process used to manufacture the VCO and should be constant over different operating temperatures.
For example in a case where a prior art VCO is used in an application for clock recovery in a serial transmission system, it is important that the output frequency remain stable and constant. The output frequency is used to reconstruct a serial transmission clock signal, which in turn, is used to sample data on a serial transmission line. Distortion or variation in the VCO output frequency, and hence, the reconstructed clock signal, can lead to sampling errors, lost data, decreased throughput, or other such problems. Consequently, for these applications it is important that the VCO provide a very stable, noise free output signal having a controlled frequency.
One problem prior art VCOs need to contend with is power supply noise. Noise, especially low frequency noise, in the power supply can have a detrimental effect on the VCO's output stability. As a typical VCO draws current from a power supply, the low frequency noise with this current (or voltage), or noise from other external devices (e.g., electromagnetic interference), can affect the output frequency. Such noise typically manifests itself as jitter on the rising and falling edges of the output signal, frequency skew in the output signal, or other distortions in the fidelity of the output.
Power supplies for VCOs are carefully filtered to remove this problematic noise. While higher frequencies are somewhat easier to remove, low frequency noise has proven more difficult and more expensive (e.g., with respect to silicon area or circuit board space) to remove. In addition, in most VCOs, a large portion of their circuitry is devoted to power supply noise rejection in order to enhance the stability of the output. These solutions are partially effective, however, as applications have become more complex and as noise sensitivity has increased with increasing levels of integration, power supply noise, particularly low frequency power supply noise, remains problematic.
Another problem is the effect of differing manufacturing process "corners". Each manufactured VCO is processed in fabrication facility and is subject to the particular variables of the specific manufacturing process employed. These variables are tightly controlled in an effort to make the devices which emerge from the process as uniform as possible. However, even the most closely monitored, tightly controlled, fabrication process has some variation, from lot to lot, of the process variables. This variation leads to slight performance variation within "families" of fabricated devices. The limits of this variation are referred to in the industry as process "corners". Hence, each nominal device emerging from fabrication will have some statistical performance variation within the process corners. At times, this variation is large enough to limit the applications to which a particular device is suited. In more severe cases, the variation can intermittently disrupt a system which includes the particular device.
Another problem is the temperature dependent variation in the output frequency of prior art VCOs. Typical prior art VCOs have difficulty maintaining a stable, constant output frequency as their operating temperatures change. When temperature increases or decreases, their output frequencies tend to increase or decrease correspondingly. This frequency variation can have a detrimental effect on the application in which the VCO is used.
Thus, what is required is a circuit which solves the power supply noise problems associated with the prior art. What is required is a circuit which is not adversely affected by low frequency noise in the power supply and which exhibits higher power supply noise rejection. What is required is a circuit which maintains a more constant, non-varying output frequency over differing operating temperatures, in comparison to the prior art. In addition, what is required is a circuit which maintains a constant and stable output frequency across the process corners. The present invention provides a novel solution to the above requirements.